Overcharge tolerant high-temperature cells and batteries

ABSTRACT

In a lithium-alloy/metal sulfide high temperature electrochemical cell, cell damage caused by overcharging is avoided by providing excess lithium in a high-lithium solubility phase alloy in the negative electrode and a specified ratio maximum of the capacity of a matrix metal of the negative electrode in the working phase to the capacity of a transition metal of the positive electrode. In charging the cell, or a plurality of such cells in series and/or parallel, chemical transfer of elemental lithium from the negative electrode through the electrolyte to the positive electrode provides sufficient lithium to support an increased self-charge current to avoid anodic dissolution of the positive electrode components above a critical potential. The lithium is subsequently electrochemically transferred back to the negative electrode in an electrochemical/chemical cycle which maintains high self-discharge currents on the order of 3-15 mA/cm 2  in the cell to prevent overcharging.

CONTRACTUAL ORIGIN OF THE INVENTION

The United States government has rights in this invention under ContractNo. W-31-109-ENG-38 between the U.S. Department of Energy and ArgonneNational Laboratory.

BACKGROUND OF THE INVENTION

This invention relates generally to the re-charging of electrochemicalcells having lithium-alloy negative electrodes and is particularlydirected, but not limited, to protecting a high temperaturelithium-alloy/metal sulfide cell and batteries from damage due toovercharging.

Lithium-alloy/metal sulfide cells are characterized by high storagecapacity and power capability per unit weight. The individual cells aretypically of the high-temperature type and are coupled in series and/orparallel to form batteries for the storage of electric power.

Re-charging of lithium-alloy/metal sulfide cells is typically carriedout in a region where the voltage of the negative electrode ismaintained at a constant, well defined level. If this voltage level isexceeded, the positive electrode may reach a higher potential conditionin which iron chloride is formed, where the positive active material orcurrent collector is an iron-based material, with the iron chloride thendissolving in the electrolyte. The dissolved iron chloride is thentransferred via the electrolyte to the negative electrode or to theseparator area resulting in destruction of the positive electrode and ashorting condition within the cell when the iron precipitate bridges thetwo electrodes. These conditions caused by cell overcharging will resultin the destruction of the cell. With a plurality of lithium alloy/metalsulfide cells coupled together to form a battery, overcharge of the weakcells of the battery is a serious problem whenever there is a disparityin the utilizable capacities of the cells. Maintaining the chargevoltage at or below a permissible voltage limit that does not produceany harmful effects on cell cycle life while allowing for thesimultaneous attainment of equal capacity in each serially connectedcell of a metal sulfide battery, cell capacity equalization, over manycycles are difficult tasks. Various approaches have been adopted toavoid cell overcharging and prevent cell damage and short circuit.

One prior art approach for battery overcharge protection makes use ofLi₂ S in the positive electrode to provide a chemical overchargetolerance by a polysulfide shuttle mechanism. The polysulfide shuttlemechanism is limited to cells having disulfide, e.g., FeS₂ or NiS₂positive electrodes and is not applicable to lithium-alloy/monosulfidecells. U.S. Pat. No. 4,324,846 to Kaun et al utilizes a ternary alloy ofiron-aluminum-lithium or nickel-aluminum-lithium orcobalt-aluminum-lithium to provide a specific overcharge capacity toafford a limited overcharge protection. Electric overcharge protectionhas also been employed such as taught in U.S. Pat. Nos. 4,079,303 to Coxand 4,238,721 to DeLuca et al. These patents disclose electrical systemsfor charging multicell storage batteries in a manner which preventsindividual cell overcharging. The former patent removes any cell fromthe charging cycle which reaches a predetermined charge voltage limit,while the latter equalizes the charge of each individual cell at aselected full charge voltage by shunting current around any cell havinga voltage exceeding this selected voltage.

The present invention does not employ extra electrical circuitry toprevent detrimental overcharge of the cell. The present invention isparticularly adapted for preventing overcharge of a battery comprised ofa plurality of lithium-alloy/metal sulfide cells by electrochemicalmeans.

OBJECTS AND SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide animproved electrochemical cell.

It is another object of the present invention to prevent overchargedamage in a high temperature electrochemical cell.

Yet another object of the present invention is to increase theself-discharge characteristics of a lithium-alloy/metal sulfide cellduring re-charging as the cell approaches the fully charged state inorder to avoid detrimental overcharging of the positive electrode.

A further object of the present invention is to provide for improvedre-charging of a battery comprised of a plurality of high temperaturelithium-alloy/metal sulfide cells connected in series and/or parallel.

A still further object of the present invention is to control theself-discharge rate of a high temperature lithium-alloy/metal sulfidecell during re-charging by providing a selected sequence ofelectrochemical phase transformations as determined by the capacity ofthe negative electrode matrix metal in the working, or operating, phaseand its potential capacity in the high lithium activity phase.

Another object of the present invention is to increase the reliabilityand operating lifetime of a high temperature lithium-alloy/metal sulfideelectrochemical cell.

A further object of this invention is to utilize chemical overchargeprotection by a lithium shuttle for fully charged cells in a battery ofcells while the charge of the battery is being continued.

Another objective of this invention is to use economical, inexpensivepositive current collector metals by allowing better control of themaximum potential of the positive electrode.

This invention contemplates a lithium-alloy/metal sulfide cell withovercharge protection comprising: a transition metal sulfide firstelectrode (positive electrode), wherein the transition metal has acapacity C_(TM) (C_(TM) =PUNC+PCYC+POCC); a lithium alloy secondelectrode (negative electrode), wherein the lithium has a capacity C_(L)(C_(L) =NUNC+NCYC+NOCC), and wherein the second electrode ischaracterized by a high lithium activity phase (richer in lithium thanin the normal cycled range) as full cell charge is approached; and anelectrolyte disposed between the first and second electrodes and havinghigh lithium solubility to allow for chemical transfer of lithium fromthe first to the second electrode during charging and an increasedself-discharge current from the second to the first electrode inpreventing overcharging of the first electrode.

BRIEF DESCRIPTION OF THE DRAWINGS

The appended claims set forth those novel features which characterizethe invention. However, the invention itself, as well as further objectsand advantages thereof, will best be understood by reference to thefollowing detailed description of a preferred embodiment taken inconjunction with the accompanying drawings, where like referencecharacters identify like elements throughout the various figures, inwhich:

FIG. 1 is a simplified schematic diagram of an overcharge toleranthigh-temperature lithium-alloy/metal sulfide cell in accordance with thepresent invention;

FIG. 2 is a graphic illustration of the variation of cell voltage andelectrode potentials during discharge and charge with changes in thedegree of charge of a lithium-alloy/metal sulfide cell in accordancewith the principles of the present invention;

FIG. 3 illustrates the capacity ranges and the critical potentials ofthe positive and negative electrodes for a lithium-alloy/metal sulfidecell in accordance with the present invention where the capacity rangesare shown in proportion to the areas of the marked fields; the upperpart of the figure shows the variation of the coulombic efficiency whenthe charge progresses from the cycled range to the overcharge range;

FIG. 4A-4F is a graphical representations of the advantageous anddisadvantageous combinations of the capacity ranges of electrodes in acell; FIG. 4F shows that the relationship specified by y between thequantities of aluminum and the other matrix metal (Si) determines thecapacity ratios of the NUNC, NCYC and NOCC sections;

FIG. 5 is a graphic representation of the variation of the difference inpositive and negative electrode potentials in terms of chargeprogression during charge and overcharge in a lithium-alloy/metalsulfide cell in accordance with the present invention; and

FIG. 6 is a graphic comparison of charge current level (I) andself-discharge rate (SDR) at different stages of lithium-alloy/metalsulfide cell charge in accordance with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention contemplates the use of electrolyte dissolved Li⁰for overcharge protection in a lithium-alloy/metal chalcogenide cell.The cell is in the form of three layers as shown in simplified schematicdiagram form in FIG. 1. In one embodiment, the negative electrode in theoperating phase is comprised of LiAlSi, while the positive electrode inthe operating phase is comprised of a metal chalcogenide. An immobilizedelectrolyte comprised of a lithium containing molten salt is disposedbetween the electrodes.

Although various metal chalcogenides such as NbSe, V₂ 0₅ can be selectedfor use in the positive electrode, in a preferred embodiment, metalsulfide comprised of FeS, Li₂ FeS₂, NiS, NiS₂, CoS or CoS₂, or a mixtureof them is selected. The immobilized electrolyte in a preferredembodiment is comprised of a mixture of MgO powder and molten salt,which is, in turn, a mixture of either 22 m % LiF-31 M % LiCl-47 m %LiBr or 25 m % LiCl-37 m % LiBr-38 m % KBr salts. The negative electrodeas fabricated in a preferred embodiment is comprised of Li_(x) Al orLi_(x) (Al_(y) Si_(1-y)) where

    0<y<1

    and

    X≦[1.13y+4(1-y)].

Lithium may also be supplied in the positive electrode as Li₂ S, if thatis desired for ease of fabrication. The total lithium in the electrodesas Li_(x) (Al_(y).Si_(1-y)) and Li₂ S must total at least l.13y+4(1-y).

In the formula of Li_(x) (Al_(y) Si₁ -y), the parenthetical part showsthe composition of the matrix metal in the negative electrode. Also,other matrix metals than Al-Si are available for use according to thisinvention such as AlFe, AlNi, AlCo, or Al or Si alone if y is one of thetwo extremes in the above inequality. In addition to the charge currentdensity the composition of the matrix metal controls the potential ofthe negative electrode during the overcharge protecting lithium shuttle(ε₅), thereby providing a means of design control for ε₅ and options ofLi° flux intensity and immobilizing materials in the electrolyte. Themelting point of the high lithium activity phase at potential ε₅ isanother important design parameter. The melting point associated withthe potential ε₅ must not be lower than the temperature of the celloperation. Consequently, according to this invention the melting pointof the electrolyte and the temperature of the cell operation must beadjusted to the properties of the high lithium activity phase of ε₅potential. The following example shows this principle. According to thisinvention, another useful negative electrode material that produces thehigh lithium activity phase at the end of the charge is a B+γ phase LiAlalloy (50-70 at % Li). This alloy can be used at lower operatingtemperatures of the battery (permitted by the melting point of theelectrolyte). For example, an Li₀.6 Al₀.4 alloy is applicable safelywithout melting up to 400° C. and an Li₀.68 Al₀.32 alloy may be used upto 330° C. Use of the LiAl alloy in overcharge proof cells requireselectrolytes of low melting point, e.g., LiCl-LiBr-KBr (m.p. 310° C.) orLiBl-KBr-CsBr (m.p. 238° C.

Previous experiments have shown that lithium alloys equilibrate with anelectrolyte containing molten lithium salts resulting in the release ofLi° into the melt. This solution is a powerful reducing agent which inthe cell reacts with the positive electrode to cause self-discharging ofthe cell. The lithium solution has a considerable electronicconductivity for increased cell self-discharge. The closer the potentialof the lithium alloy to that of pure lithium, the greater is the Li°concentration in the electrolyte and the more intense is theself-discharge of the cell due to an enhanced Li° transport rate to thepositive electrode. Although these phenomena are generally considereddisadvantageous for cell operation because it lowers current efficiency,the present invention makes use of a controlled, variable self-dischargerate to provide a chemical overcharge protection mechanism whilesacrificing little in the way of coulombic efficiency. Previous celldesigns and charge operations tried to prevent formation of anydissolved overcharge products by limiting the charge cell voltage.According to this invention the high concentration of the electrolytedissolved Li° is preferred. Furthermore, the cell voltage control is notof high importance because the cell equalization is executed primarilyunder current control conditions. Consequently, the battery of thisinvention requires only simple standard charges.

Lithium alloys at low potentials (at about -170V and below vs. Li-Alreference electrode) can maintain a sufficiently high concentration ofthe dissolved lithium to be utilized in the chemical overchargeprotection of the present invention. If the cell is designed so thatthis condition of the negative electrode is reached only at the end ofthe charge half-cycle and before the positive electrode reaches thecritical high detrimental potential (ε₂), a useful process can beimplemented. This process is based on any one of the followingreactions.

    2FeS+2Li°→Li.sub.2 FeS.sub.2 +Fe             Eq. 1

    or

    Li.sub.2 FeS.sub.2 +2Li°→Fe+2Li.sub.2 S      Eq. 2

    or

    FeS.sub.2 +2Li°→Li.sub.2 FeS.sub.2           Eq. 3

The chemical reactions set forth in Equations 1, 2 and 3 counterbalancethe anodic and cathodic formation of the electrode active materials asset forth in Equations 4, 5 and 6 and described below. Consequently, theassociated electrochemical-chemical cycle, the lithium shuttle mechanism(LSM), maintains a steady state in the cell in spite of the continuedcharge current and prevents overcharging of the positive electrode. Thisbasic principle of the chemical overcharge protection of metal sulfidecells by the lithium shuttle mechanism was engineered into a cell designas described in the following paragraphs.

In describing in detail the present invention in terms of the capacityrelationships of the positive and negative electrodes, several newcapacity terms are used in the following discussion. Each capacity termis associated with a certain well-defined section on thecharge/discharge curves as shown, for example, in FIGS. 2 and 3. Eachsection on the charge/discharge curves plays an important, distinct rolein cell operation. The lithium alloy electrode, or negative electrode,has three sections: (1) unused capacity (NUNC); (2) cycled capacity(NCYC); and (3) overcharged section (NOCC). The NUNC term represents theα-Al and (1:1:1) LiAlSi phases of the alloy; these are normally notutilized in cycles because of their poor electrode kinetics. The NOCCsection is in the range in which the alloy reaches a more negativepotential as evidenced in the negative electrode potential curve in FIG.2. In this high lithium activity condition the lithium alloy electrodereleases Li° into the electrolyte at a rate sufficient to set up aneffective lithium shuttle mechanism. Similarly, for a positive electrodein an overcharge tolerant cell in accordance with the present inventionthree operational sections can be assigned: the unused capacity section(PUNC), which is normally not utilized; the cycled capacity section(PCYC); and the overcharge section (POCC). The aforementioned terms forthe positive electrode should be understood in terms of theirrelationship to the aforementioned capacity sections of the negativeelectrode as shown in FIGS. 3 and 4A-4F.

FIGS. 4A-4F illustrate advantageous and disadvantageous combinations ofcapacity ranges with FIGS. 4A and 4B showing good overcharge protectionand high specific energy, FIG. 4C showing good overcharge protection andlower specific energy, FIG. 4D showing good overcharge protection andFIG. 4E showing no overcharge protection. FIG. 4F shows the relationshipspecified by y between the quantities of aluminum and other matrix metalwhere y₃ >y₂ >y₁, particularly in the value [1.13y+4(1-y)] discussedabove.

The POCC section, which is based on the Li₂ S and Fe content of thepositive electrode that must be in excess to the PCYC composition,provides lithium ions to overcharge the negative electrode for effectingthe lithium shuttle mechanism while being oxidized anodically. Thecorresponding positive electrode reactions are set forth in Equations 4and 5 as follows:

    Fe+2Li.sub.2 S=Li.sub.2 FeS.sub.2 +2Li.sup.+ +2e.sup.-     Eq. 4

    and

    Fe+Li.sub.2 FeS.sub.2 =2FeS+2Li.sup.+ +2e.sup.-            Eq. 5

The reaction taking place at the negative electrode is set forth inEquation 6 as follows:

    2Li.sup.+ +2e.sup.- =2Li°                           Eq. 6

The capacity ratios of these six sections determine whether overchargetolerance can exist in a cell and whether a cell has sufficient specificenergy and power as shown in FIGS. 4A-4F.

On the other hand, the capacity ratios of the six sections aredetermined and fixed by the quantities of the active material componentsused at the fabrication of the cell as described below. Usually, theNOCC and POCC sections are a little over designed to give allowance forimpurity effects and aging that render a part of the active lithiumineffective during the cycle life of the battery. Also, some additionalLi₂ S, over the quantity required by the POCC section, is advantageousto suppress dissolution of transition metal salts.

The lithium-alloy/metal sulfide cell of the present invention wasdesigned so that its positive electrode remains in the PCYC sectionduring the main part of the cycling and enters the POCC section towardsthe end of charge when the negative electrode reaches its NOCC section.These important features of the cell of the present invention aregraphically illustrated in FIGS. 2 and 3 wherein it can be seen that thepositive FeS electrode potential remains under the critical potential(ε₂) at which the irreversible anodic dissolution of the active materialwould occur, and under the potential (ε₁) at which a properly chosenpositive current collector material would be anodically dissolved. Thecritical potentials, ε₁ and ε₂, are easily measurable by a referenceelectrode, e.g., by the one described in U.S. Pat. No. 4,414,093. On theother hand, the negative LiAlSi electrode potential reaches a value (ε₅)that is adequate to maintain an effective lithium shuttle. ε₃ representsthe potential of the protected positive electrode during Li° shuttle andε₄ represents the potential of the negative electrode in the cycledrange. The three distinct operating sections of the lithium-alloy/metalsulfide cell are illustrated along the abscissa in the graph of FIG. 2as NUNC, NCYC and NOCC. The leveling curves at high Q_(m) values, whenthe cell approaches the end of the charge cycle, indicate the overchargetolerant condition maintained by the 10 mA/cm² current density of thetrickle charge current. The heavy dotted sections of the electrodepotential and charge curve represent an unwanted situation, i.e., theabsence of overcharge tolerance. This phenomena is encountered in priorart cells when the charge current is set at too high an intensity and isnot compensated for by the lithium shuttle mechanism of the presentinvention. Other prior art cells have demonstrated another basis for thelack of overcharge tolerance. In these latter prior art cells, theperformance of the positive electrode is too low to accept the Li° fluxthat otherwise might have been available for the lithium shuttlemechanism. In the middle portions of the curves shown in FIG. 2, a 100mA/cm² current density was applied during charge as well as duringdischarge.

The presence of the NUNC range is very important. Although the low cellvoltage and poor kinetics are accompanied by a decrease in cellperformance, this section is useful because it provides an overdischargecapacity as well as an indication of the approaching complete exhaustionof the negative electrode. Overdischarge tolerance of cells is importantwhen cell equalization is considered in batteries.

Investigations of LiAlSi/LiF-LiCl-LiBr/FeS cells have shown that,depending upon the conditions, 3-15 mA/cm² current density of the chargecurrent can be tolerated without overcharging the positive electrode.This current density is sufficient for charging of the cells, but doesnot represent a detrimental overcharging threat for the positive activematerial or current collector. The LiAlSi alloy is an advantageousnegative electrode material. In addition to featuring chemicalovercharge protection, the LiAlSi alloy provides chemical overdischargeindication and protection in the NUNC section by indicating the approachof the complete exhaustion of the negative electrode when thecell/battery voltage and power drop to a very low value. This sectionmay also serve as a reserve capacity.

Any negative electrode having a low-high transient step in terms of Li°dissolution in the electrolyte may be used in a lithium-alloy/metalsulfide cell in accordance with the present invention.

The practical implementation of the chemical overcharge protection ofthe present invention requires a Li° flux which is of low intensity(equivalent to <1 ma/cm²) in the normal operating range of the cell andincreases to a high intensity towards the end of charge when theovercharging protection is needed. Another important criteria incarrying out the present invention is that the charge current must notexceed the intensity of the Li° flux in the end section of the charge.These requirements can be satisfied by a combination of the propercapacity ratios (FIGS. 4-4F) and various charging techniques describedherein. A good cell design ensures high current efficiency and good cellperformance while providing overcharge tolerance.

The lower the potential of the high lithium activity phase (ε₅) thehigher the intensity of the lithium shuttle. Temperature increase alsoincreases the Li° flux. Therefore, higher temperature and lower ε₅permit a higher intensity safe cell charge equalizing current. However,both temperature and ε₅ of the equalizing process must comply with themelting point criterion of the high lithium activity phase as describedabove.

The lithium-alloy/metal sulfide cell having the charge/discharge curvesillustrated in FIG. 2 was constructed with the following parameters:

Negative electrode: Li₁.30 Al₀.86 Si₀.14, 1.54-mm thick, 115 mAh/cm²cycled capacity (NCYC).

Separator: 9.6LiF-22.0LiCl-68.4LiBr+25MgO (in wt %), 2.0-mm thick.

Positive electrode: Li₀.52 Fe₁.10 S₁.24, 1.92-mm thick, 194 mAh/cm²PUNC+PCYC+POCC capacity.

These electrode material formulas do not indicate real compounds; theymerely represent electrode compositions that are normalized to 1 mole ofthe negative matrix material (Al and Si). The electrodes were preparedby mixing LiAl and Li_(x) Si alloy powders (where x>3) for the negativeelectrode and FeS--Fe--Li₂ S powders for the positive electrode inratios that satisfy the specified formulas. It is important that thePOCC section is based on uncharged positive active materials (e.g.,Fe+Li₂ S) to supply Li⁺ ions for the lithium shuttle to overcharge thenegative electrode according to FIGS. 4A-4F, 5 and 6., i.e., the lithiumnecessary to maintain the lithium shuttle effective is stored in thePOCC section in discharged form. The capacity ranges indicated in FIG. 2are quantitated by the molar quantity of charge (Q_(m)) and are given inunits normalized to one mole of the negative matrix material (F/mol ofAl+Si).

Of the three sections shown in FIG. 2, overcharge protection is the mostimportant for the present invention. In this section, the charge curvereaches a constant cell voltage. This curve was obtained at a chargecurrent less than the highest possible self-discharge rate under theseconditions. The highest possible self-discharge rate is, in turn,related to the potential of the negative electrode and is controlled bymass-transport limitations of Li°. FIG. 2 shows a charge half-cycle inwhich the current density was reduced from 100 mA/cm² to 10 mA/cm² whenthe cell voltage reached l.63V. The cell is designed such that thecapacity ratios of the sections for the positive and negative electrodesas shown in FIG. 2 dictate a low self-discharge rate in the NCYC sectionand high rates toward the end of the charging in the NOCC section. FIG.3 illustrates the capacity ranges of the positive and negativeelectrodes and their relationships in the cell characterized by thecharge/discharge cell voltage curves illustrated in FIG. 2. Capacitiesof the ranges illustrated in FIG. 3 are in proportion to the areas ofthe marked fields.

Achieving a high cell discharge rate is facilitated by the rapiddecrease of the negative electrode potential, as shown by the potentialcurve for the LiAlSi electrode. At the same time, the potential of thepositive electrode increases only slightly and remains well below thecritical value (ε₂) which is approximately l.60V vs. a Li-Al referenceelectrode under the conditions of the experiment.

An important criterion of the effective overcharge protection is thatthe cell voltage increase is due to the negative electrode to a majorextent. The positive electrode potential may increase a little, but doesnot approach the critical potential (ε₂ or ε₁). Establishment of theimportant potentials of a cell design and setting conditions forpermissible charge methods are part of the fabrication process and areexecuted by means of a reference electrode.

The flat end section of the charge curve indicates that the cell hasreached a steady-state. The steady-state condition is brought about bythe aforementioned lithium shuttle mechanism described above in terms ofthe various aforementioned equations. By virtue of this lithium shuttlemechanism, the lithium dissolves from the overcharged negative electrodeand diffuses to the positive electrode where it is consumed by thepositive active material in a chemical reaction. The high rate of thisreaction in the NOCC section, facilitated by proper design and chargeconditions, counterbalances the anodic and cathodic formation of theelectrode active materials and terminates the charge in a steady state,before the onset of the detrimental anodic dissolution of the positiveelectrode materials. The resulting 0% coulombic efficiency of the chargecurrent under this condition of the lithium shuttle mechanism results inovercharge tolerance and provides a means for cell equalization inbatteries. Because of the nature of the lithium shuttle mechanism, thecharge current can be continued for any length of time without any neteffect on the cell if the steady state is maintained. Since theself-discharge rate in the main part of the operational range of thebattery is low and becomes high only at the end of charge during cellequalization, the coulombic efficiency of cycling is high.

According to this invention, the NOCC and POCC sections do not increasethe utilization capacity of the cell because they disappear due toself-discharge when the charge current is interrupted. This is incontrast to U.S. Pat. No. 4,324,846 that teaches the use of a specifiedutilizable overcharge capacity.

This overcharge protection mechanism is not limited to the LiAlSi/FeScell. Combinations of other Li-alloys and metal sulfides will yield atechnically exploitable Li-shuttle mechanism. In practical application,the self-discharge rate must switch from a low level during the bulk ofthe charge to a high level at the end of charge.

According to this invention the negative active material can be preparedas (1) a mixture of the lithium alloys of the matrix metals, e.g.,(Li--Al)+(Li--Si), (Li--Al)+(Li--Al--Fe), or (Li--Al)+(Li--Si)+(Al--Fe),etc., or (2) ternery or higher order alloys, e.g., (Li--Al--Si),(Li--Al--Si--Fe), or (Li--Al)+(Al--Si--Fe), etc.

There are conditions, however, when the lithium shuttle mechanism cannotkeep up with the charge current and a sharp increase in the potential ofthe positive electrode occurs toward the end of charge. This happenswhen the charge current intensity is higher than the availableself-discharge rate because either the reaction of lithium with thepositive electrode is slow or the mass-transport resistance of theseparator is too high. The upper broken line) in FIG. 2 shows theincreasing cell voltage due to the rising potential of the positiveelectrode (lower heavy broken line) when the charge current density iskept constant at 100 mA/cm².

Referring to FIG. 5, there is shown another illustration of thepotential variation of the electrodes of a lithium-alloy/metal sulfidecell in accordance with the present invention during charge andovercharge. The curves labeled N₁ and N₂ represent two versions of thenegative electrodes, with the curve labeled N₂ representing a cell whichcannot provide overcharge protection provided by the present invention.

Referring to FIG. 6, there is shown a comparison of charge current level(I) with the self-discharge rate (SDR) at different stages of charge ofthe cell. The negative electrode (N₁) protects against charge currentlevel I₁ by maintaining SDR₁ =I₁ condition at the potential ε₃ of thepositive electrode (compare to FIG. 5). The negative electroderepresented by the curve N₂ in FIG. 6 does not protect against chargecurrent level I₁. Similarly, neither of the self-discharge ratesillustrated in FIG. 6 provides overcharge protection against a chargecurrent level of I₂.

There has thus been shown a lithium-alloy/metal sulfide cell whichaffords overcharge protection by increasing its self-discharge rate by aselected sequence of electrochemical phase transformations in providingthe necessary capacity relationships of the positive and negativeelectrodes during various phases of cell charging as shown in FIG.4A-4F. In this approach, the self-discharge rate increases to a level atleast two times the level maintained at early states of charge beforethe positive electrode potential can reach a detrimentally high value(ε₁ or ε₂). In addition, the charge current level at the end of thecharge cycle is maintained equal to the increased self-discharge currentto prevent cell damage by overcharging.

While particular embodiments of the present invention have been shownand described, it will be obvious to those skilled in the art thatchanges and modifications may be made without departing from theinvention in its broader aspects. Therefore, the aim in the appendedclaims is to cover all such changes and modifications as fall within thetrue spirit and scope of the invention. The matter set forth in theforegoing description and accompanying drawings is offered by way ofillustration only and not as a limitation. The actual scope of theinvention is intended to be defined in the following claims when viewedin their proper perspective based on the prior art.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:
 1. A chemical overchargeprotection arrangement for a re-chargeable electrochemical cellcharacterized by variation of lithium content of a negative electrode ofthe cell as the cell is cycled between charged and discharged states,said arrangement comprising:a positive, first electrode; a negative,lithium alloy, second electrode, wherein said second electrode ischaracterized by a high lithium-activity phase which is richer inlithium than in the normal cycled range as full cell charge isapproached; and an electrolyte disposed between said first and secondelectrodes and having high lithium solubility to allow for chemicaltransfer of lithium from the first to the second electrode duringcharging and an increased self-discharge current from the second to thefirst electrode in preventing overcharging of said first electrode. 2.The electrochemical cell of claim 1 wherein:the positive electrodeincludes a transition metal sulfide with a transition metal having acapacity of PUNC+PCYC+POCC; and the negative electrode includes lithiumcombined with a matrix metal having a capacity of NUNC+NCYC+NOCC, withPOCC extending to a higher capacity than NOCC.
 3. The electrochemicalcell of claim 2 wherein said electrolyte is comprised of a molten salt.4. The electrochemical cell of claim 3 wherein said molten saltelectrolyte includes lithium salt.
 5. The electrochemical cell of claim4 wherein said molten salt electrolyte further includes MgO powder. 6.The electrochemical cell of claim 5 wherein said molten salt electrolyteincludes a mixture of 22 m % LiF--31 m % LiCl--47 m % LiBr salts.
 7. Theelectrochemical cell of claim 5 wherein said molten salt electrolyteincludes a mixture of Li Cl--LiBr--KBr salts.
 8. The electrochemicalcell of claim 2 wherein said positive electrode is selected from thegroup comprised of FeS, Li₂ FeS₂, FeS₂, NiS, Ni₃ S₂, NiS₂, CoS, CoS₂ andmixtures thereof.
 9. The electrochemical cell of claim 2 wherein saidpositive electrode is comprised of FeS--Fe--Li₂ S.
 10. Theelectrochemical cell of claim 2 wherein the lithium alloy in saidnegative electrode has a higher melting point than an operatingtemperature of the cell.
 11. The electrochemical cell of claim 2 whereinsaid negative electrode is comprised of Li_(x) Al.
 12. Theelectrochemical cell of claim 2 wherein said negative electrode iscomprised of lithium, aluminum and silicon.
 13. The electrochemical cellof claim 2 wherein said negative electrode is comprised of Li_(x)(Al_(y) Si_(1-y)) alloy, where 0<y<1 and x≦[1.13y+4(1-y)] and additionallithium is included as Li₂ S in the positive electrode to provide atotal lithium amount in these forms of at least 1.13y+4(1-y).
 14. Theelectrochemical cell of claim 2 wherein said negative electrode includesa lithium alloy selected from the group consisting of Li--Al--Fe,Li--Al--Ni, Li--Al--Co and Li--Al--Fe--Si.
 15. A method of chemicalovercharge protection for an electrochemical cell comprising:providing apositive electrode including transition metal chalcogenide with atransition metal capacity of PUNC+PCYC+POCC, a negative, lithium alloyelectrode with lithium combined with a matrix metal having a capacity ofNUNC+NCYC+NOCC with POCC extending to a higher capacity than NOCC, andan electrolyte between the positive and negative electrodes; chargingthe cell to the capacity NUNC+NCYC at a first charge rate to increasethe transfer rate of Li° from the negative electrode into theelectrolyte; and charging the cell to beyond the capacity NCYC at asecond charge rate balanced by Li° transport from the negative to thepositive electrode and by Li⁺ migration from the positive to thenegative electrode.
 16. The method of claim 15 wherein said first chargerate is more than said second charge rate.
 17. The method of claim 15wherein the charging is interrupted between the first and second chargerates.
 18. The method of claim 15 wherein said electrochemical cell isin a battery of electrochemical cells and cells of lower charge arepermitted to reach full charge and fully charged cells are protected.